1. Technical Field
The present invention relates to methods of manufacturing stator coils for electric rotating machines that are used in, for example, motor vehicles as electric motors and electric generators.
2. Description of the Related Art
There are known stators for electric rotating machines which include a hollow cylindrical stator core and a stator coil. The stator core has a plurality of slots that are formed in the radially inner surface of the stator core and spaced at equal intervals in the circumferential direction of the stator core. The stator coil is comprised of a plurality of electric wires mounted on the stator core. Each of the electric wires includes a plurality of in-slot portions and a plurality of turn portions. Each of the in-slot portions is received in a corresponding one of the slots of the stator core. Each of the turn portions is located outside of the slots of the stator core and connects one adjacent pair of the in-slot portions of the electric wire.
Moreover, there is disclosed, for example in Japanese Patent Application Publication No. 2001-145286, a method of manufacturing a stator. According to the method, to improve the space factors of the electric wires in the slots of the stator core, each of the electric wires forming U-phase, V-phase, and W-phase windings of the stator coil is configured to have a rectangular cross section perpendicular to its longitudinal axis and have such an overall shape that when developed on a plane, the electric wire meanders in the form of cranks. Further, the stator coil is formed by: (1) stacking the electric wires together to form a planar electric wire assembly; and (2) rolling the planar electric wire assembly by a predetermined number of turns into a hollow cylindrical shape.
For the thus-formed stator coil, it is necessary for those corresponding in-slot portions of the electric wires which are to be received in the same slot of the stator core to be aligned in a radial direction of the stator coil. However, due to springback of the electric wires which are only elastically deformed during the rolling step, it may be easy for radial misalignment between the corresponding in-slot portions of the electric wires to occur, rendering it difficult to keep the hollow cylindrical shape of the stator coil. Consequently, it may be difficult to easily and accurately assemble the stator coil and the stator core.
Further, to secure higher space factors of the electric wires in the slots of the stator core, it is preferable to more densely arrange the in-slot portions of the electric wires in the corresponding slots of the stator core. However, in this case, insulating coats formed at the surfaces of the in-slot portions of the electric wires may be damaged due to friction which may occur between adjacent pairs of the in-slot portions of the electric wires during the rolling step. Consequently, it may be difficult to ensure electrical insulation between the in-slot portions of the electric wires.
Moreover, with the above method, each of the electric wires is required to have a long length. Accordingly, a large-scale shaping machine is needed for shaping the electric wires. In addition, with the long length of the electric wires, it may be difficult to handle the electric wires during the manufacture of the stator. As a result, it may be difficult to secure a high productivity and a low cost of the stator.